Direct incorporation of ethanol into diesel fuel is currently limited owing to the low miscibility of ethanol and of a diesel base fuel. Mixtures are thus limited to an incorporation ratio of about 3% by volume of ethanol, or they require the use of cosolvents. Patent application FR-A-2,895,418 filed on behalf of the assignee Institut Francais du Petrole proposes using vegetable oil methyl esters (VOME) as the cosolvent. Thus, the diesel fuel composition described in this patent application allows the incorporation of 10 to 30% by volume of ethanol into a diesel base fuel. Furthermore, a variant consisting of highly hydrotreated oils improves the cetane number otherwise decreased by the presence of ethanol.
Another solution considered for increasing the proportion of ethanol that can be incorporated into diesel fuel consists in using a surfactant additive permitting the ethanol to be suspended in the diesel fuel, thus creating a microemulsion.
However, in some cases, the presence of ethanol in mixtures significantly modifies the values of the flash point, defined as the lowest temperature at which the concentration of the vapours emitted is sufficient to produce a deflagration on contact with a flame or a hot spot, but remains insufficient for self-ignition, a spontaneous combustion in the absence of “pilot” flame. The flash point thus serves to classify liquids into particular categories according to their ignition risks. One thus distinguishes class I flammable liquids (flash point below 55° C.) and class II flammable liquids (flash point above 55° C.). The storage, handling and distribution conditions are of course different depending on the class to which the fuel belongs.
The table below (Table 1) gives the flash point values of some products:
TABLE 1ProductsFlash pointFuel categorygasoline<−25° C. Class Ikerosene>38° C.Class Ipetroleum-based diesel fuel>55° C.Class IIethanol~13° C.Class Irapeseed methyl ester>120° C. Class II
Now, the flash point of mixtures is imposed by the most volatile product, from the moment that its presence reaches some percents (Hansen A. C. et al., Bioresource Technology, 2005 96 (3) 277). Thus, incorporation of some percents of ethanol into mixtures rapidly leads to reduce the flash point of the mixtures and brings it to that of ethanol, i.e. around 13° C. Diesel fuels in admixture with ethanol therefore no longer belong to class II, they are comparable to class I fuels.
However, the flash point values reached are critical and the use of ethanol in diesel fuel may lead, above the liquid, to the existence of a flammable system: these mixtures are then located in risk zones as regards explosiveness. In fact, in the case of gasolines with a very low flash point, the gas overhead in the vehicle gaseous tank consists of an air-fuel mixture that is too rich to ignite. In diesel vehicles, considering the low volatility of diesel fuel, the gas overhead consists on the contrary of too lean a mixture. One solution when the mixtures are based on diesel fuel and ethanol consists in providing all vehicle tanks and storage tanks with a sealed system and a flame arrester equipment. This solution has the drawback of involving modifications and adjustments of the vehicle itself.
A solution consisting in directly improving the fuel formulation so as to obtain mixtures having an acceptable flash point with the conditions of use and storage of the fuel, without requiring engine modifications, appears to be more promising. The present invention lies within this scope.